Hydraulic systems are known which utilize a single source of fluid under pressure for operating a variable displacement drive motor and an auxiliary motor. For example, U.S. Pat. No. 3,984,978 issued Oct. 12, 1976, to L. L. Alderson describes a variable displacement pump providing fluid through a control system for operating a pair of variable displacement drive motors and also for operating auxiliary equipment. In that example, the drive motors are individually connected to the front wheels of a motor grader, and the auxiliary equipment would usually include one or more motors for powering an earthmoving implement mounted on the motor grader, such as a ripper, moldboard or the like.
Typically, in such motor grader environment, the drive motors are operated substantially continually when front wheel drive power is required. At the same time, the auxiliary equipment is operated only intermittently. As a result, the capacity of the pump is generally dictated for economic reasons on the requirements of the continually operated wheel drive motors. However, when the auxiliary equipment is operated at a time when the variable displacement dirve motors are demanding substantially all the available capacity of the pump, there is often insufficient fluid under the pressure required to operate the auxiliary equipment motor.
In the case of the motor grader control system described in aforementioned U.S. Pat. No. 3,984,978, for example, a pressure compensating spool is incorporated to establish a minimum pressure level for operation of the pair of front wheel drive motors. If the speed of the vehicle is so high as to demand more flow capacity from the pump than is available, then such pressure compensating spool automatically acts through this system to destroke the motors and maintain at least that pressure level. Unfortunately, simultaneous operation of an auxiliary motor would be limited to that same preestablished minimum pressure level, even though a higher pressure is required for satisfactory performance of the loaded auxiliary motor.
The present invention is directed to overcoming one or more of the problems as set forth above.